This invention relates to test fixtures used in testing RF transistors in microwave frequencies (see ref. 1, 2 and 3). Active RF components (transistors—DUT) need to be thoroughly tested at the operation frequencies before used in amplifier and other circuit designs. Regular transistor test fixtures come in two basic forms: Micro-strip fixtures (FIGS. 1 and 3) or coaxial fixtures (see ref. 1 and 5).
Transistor test fixtures usually comprise an input (2) and an output (4) block, on which’ external ports in form of coaxial connectors (1, 5) are attached. The blocks can be cascaded (3) directly in order for their internal ports (9, 10) to meet and the fixture to be calibrated (when the INSERT (8) is not used) and the measured data corrected back (de-embedded) to the transistor (7, DUT) reference plane (FIG. 2), or they can be separated, in order to insert a block (8, INSERT) which carries the DUT (7) (FIG. 1). The signal is guided through micro-strip lines (6) or coplanar waveguide lines (see ref. 4) leading to the transistor (21) input (22) and output (24) terminals. The transistor package (21, 7) is normally grounded (23) on the INSERT (8) using clamps or screws.
The problem with this structure is shown in FIG. 4b). The soft dielectric substrate (43) normally subsides to the pressure of the screws (44) holding it in place (also shown in FIG. 3 (39)) and bend, allowing cavities (41) to be formed between the conductively coated (metallized) substrate bottom and the top of the metal block (45). These cavities often create undesired random resonances (see FIG. 8, item (81)), which make the fixture useless.